libbacktrace: Fix typos in various files

Signed-off-by: Dhruv Chawla <dhruvc@nvidia.com>

libbacktrace/ChangeLog:

	* allocfail.sh: Fix typos.
	* elf.c (elf_fetch_bits): Likewise.

allocfail.sh

@@ -77,7 +77,7 @@ while [ $i -le $allocs ]; do
 	exit 1
     fi
 
-    # The test-case would run too long if we would excercise all allocs.
+    # The test-case would run too long if we would exercise all allocs.
     # So, run with step 1 initially, and increase the step once we have 10
     # subsequent passes, and drop back to step 1 once we encounter another
     # failure.  This takes ~2.6 seconds on an i7-6600U CPU @ 2.60GHz.

elf.c

@@ -1166,7 +1166,7 @@ elf_fetch_bits (const unsigned char **ppin, const unsigned char *pinend,
   return 1;
 }
 
-/* This is like elf_fetch_bits, but it fetchs the bits backward, and ensures at
+/* This is like elf_fetch_bits, but it fetches the bits backward, and ensures at
    least 16 bits.  This is for zstd.  */
 
 static int