Fix a few typos

Reviewed-by: Francisco Jerez <currojerez@riseup.net>
diff --git a/src/glx/glxcmds.c b/src/glx/glxcmds.c
index f4f476e..26ff804 100644
--- a/src/glx/glxcmds.c
+++ b/src/glx/glxcmds.c
@@ -496,7 +496,7 @@
 }
 
 /*
-** Query the existance of the GLX extension
+** Query the existence of the GLX extension
 */
 _X_EXPORT Bool
 glXQueryExtension(Display * dpy, int *errorBase, int *eventBase)