Fixup of 'Fix spelling of `ADITIONAL_VENV_PACKAGES`'

Introduced a dangling } in the environment variable.
This removes it.

Change-Id: If9413dc1751399e5b9c9a0094772394252e5a81c
diff --git a/stackrc b/stackrc
index 6cbadf9..59ba907 100644
--- a/stackrc
+++ b/stackrc
@@ -209,7 +209,7 @@
 # Add packages that need to be installed into a venv but are not in any
 # requirements files here, in a comma-separated list.
 # Currently only used when USE_VENV is true (individual project venvs)
-ADDITIONAL_VENV_PACKAGES=${ADDITIONAL_VENV_PACKAGES:-""}}
+ADDITIONAL_VENV_PACKAGES=${ADDITIONAL_VENV_PACKAGES:-""}
 
 # This can be used to turn database query logging on and off
 # (currently only implemented for MySQL backend)