docs(plugins): change the example's minimum poetry version to 1.2

this is the first poetry version with plugin support, so plugins
cannot be compatible with any version before 1.2.

(cherry picked from commit 8a500f0)

Author: onerandomusername

docs/plugins.md

@@ -39,7 +39,7 @@ version = "1.0.0"
 # ...
 [tool.poetry.dependencies]
 python = "^3.7"
-poetry = "^1.0"
+poetry = "^1.2"
 
 [tool.poetry.plugins."poetry.plugin"]
 demo = "poetry_demo_plugin.plugin:MyPlugin"