Fix typo in MODULE_BUILD_FROM_SOURCE variable.

Cherry-picked from http://ag/15300830.

Test: env MODULE_BUILD_FROM_SOURCE=true \
        TARGET_PRODUCT=cf_x86_64_phone build/soong/soong_ui.bash \
        --dumpvar-mode SOONG_CONFIG_art_module_source_build
  returns true
Test: env TARGET_PRODUCT=cf_x86_64_phone build/soong/soong_ui.bash \
        --dumpvar-mode SOONG_CONFIG_art_module_source_build
  returns false
Bug: 191978129
Change-Id: I2056b198bc0b944a6736ff21077b902df13c1479
Merged-In: I2056b198bc0b944a6736ff21077b902df13c1479