Change source tag to use _ (#446)

## Problem

Source tags need to use _ not -.

## Solution

Describe the approach you took. Link to any relevant bugs, issues, docs,
or other resources.

## Type of Change

- [x] Bug fix (non-breaking change which fixes an issue)
- [ ] New feature (non-breaking change which adds functionality)
- [ ] Breaking change (fix or feature that would cause existing
functionality to not work as expected)
- [ ] This change requires a documentation update
- [ ] Infrastructure change (CI configs, etc)
- [ ] Non-code change (docs, etc)
- [ ] None of the above: (explain here)

## Test Plan

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Author: jennapederson

docs/semantic-search.ipynb

@@ -131,7 +131,7 @@
         "pc = Pinecone(\n",
         "        # You can remove this for your own projects!\n",
         "        api_key=api_key,\n",
-        "        source_tag=\"pinecone_examples:docs:semantic-search\"\n",
+        "        source_tag=\"pinecone_examples:docs:semantic_search\"\n",
         "    )"
       ]
     },