docs: typo (fixes #1325)

Add curly braces around initial value of a key that contains an equal sign "=".

Signed-off-by: Yukai Chou <muzimuzhi@gmail.com>

Author: muzimuzhi

doc/generic/pgf/pgfmanual-en-library-calendar.tex

@@ -261,7 +261,7 @@ \subsection{Calendar Command}
 \end{codeexample}
     \end{key}
 
-    \begin{key}{/tikz/every day (initially anchor=base east)}
+    \begin{key}{/tikz/every day (initially {anchor=base east})}
         This style is executed by the default node code for each day. The
         |every day| style is useful for changing the way days look. For
         example, let us make all days red: