[gdb/testsuite] Fix regexp in gdb.dwarf2/dw2-inter-cu-error.exp

In commit b5070480d74 ("[gdb/symtab] Change DWARF_ERROR from Dwarf Error to
DWARF Error") I changed the dwarf error prefix, but failed to update test-case
gdb.dwarf2/dw2-inter-cu-error.exp.

Fix this by updating the corresponding regexp in the test-case.

Tested on x86_64-linux.

gdb/testsuite/gdb.dwarf2/dw2-inter-cu-error.exp

diff --git a/gdb/testsuite/gdb.dwarf2/dw2-inter-cu-error.exp b/gdb/testsuite/gdb.dwarf2/dw2-inter-cu-error.exp
index a628650..92ffcae 100644
--- a/gdb/testsuite/gdb.dwarf2/dw2-inter-cu-error.exp
+++ b/gdb/testsuite/gdb.dwarf2/dw2-inter-cu-error.exp
@@ -47,5 +47,5 @@
 }
 
 # Verify that GDB notices the null DIE.
-gdb_assert { [regexp "Dwarf Error: Unexpected null DIE" $gdb_file_cmd_msg] } \
+gdb_assert { [regexp "DWARF Error: Unexpected null DIE" $gdb_file_cmd_msg] } \
 	"Null DIE error missing"