[Docs] update branch version to 1.3 (#1333)

`branch` in the package.json is used for pointing links to the right
site. For example: `https://opensearch.org/docs/1.3/dashboards/index/`.

NOTE: At the time this commit was made the doc site for `1.3`, is
not available.

There is an existing issue here:
opensearch-project/documentation-website#296

We can point this to `latest` but the problem with that is that we
don't re-release versions. When the next major or minor release
occurs then `latest` will point to that version and now all
OpenSearch Dashboards 1.3 downloads will point to the newest version.

So to avoid that, I suggest we point this to `1.3` and I got confirmation
that it will be `1.3`.

Issue resolved:
n/a

Signed-off-by: Kawika Avilla <[email protected]>

package.json

@@ -12,7 +12,7 @@
   ],
   "private": true,
   "version": "1.3.0",
-  "branch": "1.2",
+  "branch": "1.3",
   "types": "./opensearch_dashboards.d.ts",
   "tsdocMetadata": "./build/tsdoc-metadata.json",
   "build": {