DOC: fix xref and rename api changes to correct scheme

Author: tacaswell

…/next_api_changes/behavior/99999-tac.rst …/next_api_changes/behavior/20150-TAC.rstdoc/api/next_api_changes/behavior/99999-tac.rst renamed to doc/api/next_api_changes/behavior/20150-TAC.rst doc/api/next_api_changes/behavior/99999-tac.rst renamed to doc/api/next_api_changes/behavior/20150-TAC.rst

@@ -4,7 +4,7 @@ Rename fist arg to subplot_mosaic
 Both `.FigureBase.subplot_mosaic`, and `.pyplot.subplot_mosaic` have had the
 first position argument renamed from *layout* to *mosaic*.  This is because we
 are considering to consolidate *constrained_layout* and *tight_layout* keyword
-arguments in the Figure creation functions of `pyplot` into a single *layout*
+arguments in the Figure creation functions of `.pyplot` into a single *layout*
 keyword argument which would collide.
 
 As this API is provisional, we are changing this with no deprecation period.