README: format install command
README: format install command

Separate command into a separate line in order to make it easier to copy and paste into the terminal

Change-Id: I2e446475201c40ddb5054fe403ff23ed79b0742e
GitHub-Last-Rev: bbd650aa12415725168b59ec934aafeeaf0f21aa
GitHub-Pull-Request: golang/tools#346
Reviewed-on: https://go-review.googlesource.com/c/tools/+/361394
Auto-Submit: Bryan Mills <1c8aad60184261ccede67f5c63a0d2a3bf3c9ff4@google.com>
Reviewed-by: Robert Findley <9179eee875d1ee628c37fb47d7e749e855ffd21f@google.com>
gopls-CI: kokoro <2ac7b1f3fa578934c95181d4272be0d3bca00121@google.com>
Reviewed-by: Cherry Mui <d62e63aa42ce272d7b6a5055d97e942b33a34679@google.com>
Reviewed-by: Bryan Mills <1c8aad60184261ccede67f5c63a0d2a3bf3c9ff4@google.com>
Run-TryBot: Bryan Mills <1c8aad60184261ccede67f5c63a0d2a3bf3c9ff4@google.com>

golang/tools,golang/tools,golang/tools,golang/tools,golang/tools