[lldb] Fix format string in FindPrologueSize

[JDevlieghere]

The format string was using an incorrect index (1 instead of 0) which was caught by an asserts build.

rdar://166562512

[JDevlieghere]

@swift-ci test

[felipepiovezan]

01:34:54  TIMEOUT: lldb-shell :: Breakpoint/jit-loader_rtdyld_elf.test (2501 of 2501)
01:34:54  ******************** TEST 'lldb-shell :: Breakpoint/jit-loader_rtdyld_elf.test' FAILED ********************
01:34:54  Exit Code: 1
01:34:54  Timeout: Reached timeout of 900 seconds

[felipepiovezan]

@swift-ci test windows platform

[felipepiovezan]

Thanks for fixing my little slip up :)

[JDevlieghere]

@swift-ci test windows platform