Replace magic number in CFE_FS_WriteHeader with CFE_FS_FILE_CONTENT_ID

[skliper]

Is your feature request related to a problem? Please describe.
Magic number:

cFE/modules/fs/fsw/src/cfe_fs_api.c

Lines 195 to 196 in 73c338d

	/* put the header, 'cfe1' in hex, in to the content type */
	Hdr->ContentType = 0x63464531;

Describe the solution you'd like
There's already a define, use it:

cFE/modules/core_api/fsw/inc/cfe_fs_extern_typedefs.h

Line 53 in 73c338d

#define CFE_FS_FILE_CONTENT_ID 0x63464531 /**< \brief Magic Number for cFE compliant files (= 'cFE1') */

Describe alternatives you've considered
None

Additional context
Code review

Requester Info
Jacob Hageman - NASA/GSFC