Replace solidus -> slash

Author: ppkarwasz

PURL-SPECIFICATION.rst

@@ -415,7 +415,7 @@ To parse a ``purl`` string in its components:
   - Discard any empty segment from that split
   - Percent-decode each segment
   - UTF-8-decode each segment if needed in your programming language
-  - Report an error if any segment contains a solidus `/`
+  - Report an error if any segment contains a slash `/`
   - Apply type-specific normalization to each segment if needed
   - Join segments back with a '/'
   - This is the ``namespace``