Fix YAML exception raised by jekyll on tools/plugins.md.template

Quote the permalink attribute in the jekyll header of this file to
fix the exception raised without quotes:

"YAML Exception reading /site/tools/plugins.md.template: (<unknown>):
did not find expected key while parsing a block mapping at line 2
column 1"

Jekyll/Liquid uses double curly braces for templating. However, in YAML,
curly braces { } also denote an inline mapping (JSON-style dictionary).
When the YAML parser sees {{, it thinks this is the start of a
dictionary but finds the syntax invalid because there is no key-value
pair inside the braces that fits YAML standards.

Change-Id: I1c22fe28a2e3d711fdee829adbf239626df28c3c