Update DOCDIR path to the final installation path

After we have changed the docdir location on Windows
for Geany in geany/geany@f4b270a
do this also for Geany-Plugins.

build/vars.build.mk

@@ -2,7 +2,7 @@ if MINGW
 LOCAL_AM_CFLAGS = \
 	-DLOCALEDIR=\""share/locale"\" \
 	-DPREFIX=\"\" \
-	-DDOCDIR=\""doc/geany-plugins"\" \
+	-DDOCDIR=\""share/doc/geany-plugins"\" \
 	-DGEANYPLUGINS_DATADIR=\""share"\" \
 	-DPKGDATADIR=\""share/geany-plugins"\" \
 	-DLIBDIR=\""lib"\"