fix text context and complete gradient js code

Example/examples/Clipping.js

@@ -164,7 +164,7 @@ class TextClipping extends Component{
                 fontWeight="bold"
                 fill="red"
                 stroke="blue"
-                textAnchor="center"
+                textAnchor="middle"
                 clipPath="url(#clip)"
             >NOT THE FACE</Text>
         </Svg>;

Example/examples/G.js

@@ -84,7 +84,7 @@ class GTransform extends Component{
                     y="75"
                     stroke="#600"
                     fill="#600"
-                    textAnchor="center"
+                    textAnchor="middle"
                 >
                     Text grouped with shapes</Text>
             </G>

Example/examples/Stroking.js

@@ -76,7 +76,7 @@ class StrokeDashoffset extends Component{
                 fontWeight="bold"
                 x="100"
                 y="40"
-                textAnchor="center"
+                textAnchor="middle"
                 strokeDasharray="100"
                 strokeDashoffset="60"
             >STROKE</Text>

Example/examples/Text.js

@@ -22,7 +22,7 @@ class TextExample extends Component{
                 x="50"
                 y="9"
                 fill="red"
-                textAnchor="center"
+                textAnchor="middle"
             >I love SVG!</Text>
         </Svg>;
     }
@@ -81,7 +81,7 @@ class TextFill extends Component{
                 fontWeight="bold"
                 x="100"
                 y="20"
-                textAnchor="center"
+                textAnchor="middle"
             >FILL TEXT</Text>
         </Svg>;
     }
@@ -109,7 +109,7 @@ class TextStroke extends Component{
                 fontWeight="bold"
                 x="100"
                 y="20"
-                textAnchor="center"
+                textAnchor="middle"
             >STROKE TEXT</Text>
         </Svg>;
     }
@@ -148,7 +148,7 @@ const icon = <Svg
         y="2"
         fontSize="14"
         fontWeight="bold"
-        textAnchor="center"
+        textAnchor="middle"
         fill="none"
         stroke="blue"
     >字</Text>