Fix typos in some files (matplotlib#30759)

Co-authored-by: khanhkhanhlele <namkhanh20xx@gmail.com>

Author: Edge-Seven

extern/agg24-svn/include/ctrl/agg_gamma_spline.h

@@ -56,7 +56,7 @@ namespace agg
     // bounding rectangle. Function values() calculates the curve by these
     // 4 values. After calling it one can get the gamma-array with call gamma(). 
     // Class also supports the vertex source interface, i.e rewind() and
-    // vertex(). It's made for convinience and used in class gamma_ctrl. 
+    // vertex(). It's made for convenience and used in class gamma_ctrl. 
     // Before calling rewind/vertex one must set the bounding box
     // box() using pixel coordinates. 
     //------------------------------------------------------------------------