DOC: assorted typo fixes in the documentation

This is the third round of cleanups in various docs
diff --git a/doc/internals/buffer-api.txt b/doc/internals/buffer-api.txt
index 6d192c1..14a1ac7 100644
--- a/doc/internals/buffer-api.txt
+++ b/doc/internals/buffer-api.txt
@@ -630,7 +630,7 @@
                    <------------------> <---------->
                             o                i
 
-There is this correspondance between old and new fields (some will involve a
+There is this correspondence between old and new fields (some will involve a
 knowledge of a channel when the output byte count is required) :
 
      Old    | New