Fix PKPM SSP-RK3 stage-1 field limiter target

[jasisz]

The first PKPM SSP-RK3 stage writes its field result to em1, but the optional field limiter was called on emnew. With field.limit_em = true, stage 2 therefore consumed the un-limited em1.

This changes the stage-1 limiter target to em1, matching the stage output and the corresponding Vlasov SSP-RK3 implementation.

Validation:

• make -j8 pkpm
• focused driver harness: before the fix, the first limiter targets emnew and stage 2 receives the un-limited value 4; after the fix, the limiter targets em1 and stage 2 receives the limited value 1
• actual Maxwell limiter kernel reduces the test Ex slope from 4 to 0 while retaining its average of 1