README: fix PowerShell env var instructions (#1025)

This fixes #1024.

Author: mrnugget

README.md

@@ -144,8 +144,8 @@ SRC_ENDPOINT=https://sourcegraph.example.com SRC_ACCESS_TOKEN=my-token src searc
 Type the following on your PowerShell terminal:
 
 ```
-$env.SRC_ENDPOINT = 'https://sourcegraph.example.com'
-$env.SRC_ACCESS_TOKEN = 'my-token'
+$env:SRC_ENDPOINT = 'https://sourcegraph.example.com'
+$env:SRC_ACCESS_TOKEN = 'my-token'
 ```
 
 Replace `my-token` and `https://sourcegraph.example.com` with the appropriate values for your account and instance.