Changed "and" to "an"

Changed "This is and index/directory of ..." to "This is and index/directory of...".

[ticket: X]

Author: wclodius2

doc/specs/index.md

@@ -4,7 +4,7 @@ title: Specifications (specs)
 
 # Fortran stdlib Specifications (specs)
 
-This is and index/directory of the specifications (specs) for each new module/feature as described in the
+This is an index/directory of the specifications (specs) for each new module/feature as described in the
 [workflow document](../Workflow.html).
 
 [TOC]