Update TextSecure Staging URL instructions
Update TextSecure Staging URL instructions

Small change to clarify where the TextSecure server URL should be changed to the staging value in local builds. 

It looks like the Android variable has been updated from `TEXTSECURE_URL` to `SIGNAL_URL` (https://github.com/WhisperSystems/Signal-Android/commit/ae40715526aa0fbad583783be63115bb46b1c2c8#diff-c197962302397baf3a4cc36463dce5eaR179) and the iOS counterpart is named `textSecureServerURL`.

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