docs: fix refs to renamed \pgfdecorate (fixes pgf-tikz#1388)

`\pgfdecorate ... \endpgfdecorate` were renamed to `\pgfdecoration ...
\endpgfdecoration` in f58eddf (Decoration updates., 2008-01-14).

Signed-off-by: Yukai Chou <[email protected]>

Author: muzimuzhi

doc/generic/pgf/pgfmanual-en-base-decorations.tex

@@ -732,10 +732,10 @@ \subsection{Using Decorations}
     \meta{name}. This is equivalent to
     %
 \begin{codeexample}[code only]
-\pgfdecorate{{name}{\pgfdecoratedpathlength}
-             {\pgfdecoratebeforecode}{\pgfdecorateaftercode}}
+\pgfdecoration{{name}{\pgfdecoratedpathlength}
+               {\pgfdecoratebeforecode}{\pgfdecorateaftercode}}
   // the path commands.
-\endpgfdecorate
+\endpgfdecoration
 \end{codeexample}
     %
 \end{command}
@@ -747,11 +747,13 @@ \subsection{Using Decorations}
 Both the above commands use the current definitions of the following macros:
 
 \begin{command}{\pgfdecoratebeforecode}
-    Code executed as \meta{before code}, see the description of |\pgfdecorate|.
+    Code executed as \meta{before code}, see the description of
+    |\pgfdecoration|.
 \end{command}
 
 \begin{command}{\pgfdecorateaftercode}
-    Code executed as \meta{after code}, see the description of |\pgfdecorate|.
+    Code executed as \meta{after code}, see the description of
+    |\pgfdecoration|.
 \end{command}
 
 It may sometimes be useful to add an additional transformation for each segment